linux-kernel - Re: [PATCH] drm/panthor: add query for calibrated timstamp info

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Message-ID: <CAPaKu7RK2dW4LYbCj7ksbNJVst+Yn5aetKhJG0N=EyTY8BhYfw@mail.gmail.com>
Date: Thu, 2 Oct 2025 17:53:34 -0700
From: Chia-I Wu <olvaffe@...il.com>
To: Lukas Zapolskas <lukas.zapolskas@....com>
Cc: nd@....com, Boris Brezillon <boris.brezillon@...labora.com>, 
	Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Grant Likely <grant.likely@...aro.org>, Heiko Stuebner <heiko@...ech.de>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	marcin.slusarz@....com
Subject: Re: [PATCH] drm/panthor: add query for calibrated timstamp info

On Fri, Sep 26, 2025 at 3:41 AM Lukas Zapolskas <lukas.zapolskas@....com> wrote:
>
> Hello Chia-I,
>
[...]
> > +     switch (out->cpu_clockid) {
> > +     case CLOCK_MONOTONIC:
> > +             cpu_timestamp = ktime_get_ns;
> > +             break;
> > +     case CLOCK_MONOTONIC_RAW:
> > +             cpu_timestamp = ktime_get_raw_ns;
> > +             break;
> > +     case CLOCK_REALTIME:
> > +             cpu_timestamp = ktime_get_real_ns;
> > +             break;
> > +     case CLOCK_BOOTTIME:
> > +             cpu_timestamp = ktime_get_boottime_ns;
> > +             break;
> > +     case CLOCK_TAI:
> > +             cpu_timestamp = ktime_get_clocktai_ns;
> > +             break;
>
> Out of interest, what is the use-case for the REALTIME, BOOTTIME and TAI clocks? Looking at
> VK_KHR_calibrated_timestamps, it seems that only MONOTONIC and MONOTONIC_RAW are exposed directly.
> I worry that providing the other clocks may make it easier for accidentally querying timestamps that
> can't be correlated with driver state. A recent Mesa change aligned PanVK Perfetto instrumentation on
> MONOTONIC_RAW [1], and the performance counter patches I've proposed also use MONOTONIC_RAW
> as the only clock source.
I followed drm_xe_query_engine_cycles without giving much thought. As
long as we leave room for future extension, we can certainly only
allow those required by vulkan.